The present invention relates to homogeneous iron-based powder mixtures of the kind containing iron or steel powders and at least one alloying powder. More particularly, the invention relates to such mixtures that contain a binder of high molecular weight polyalkylene oxide that not only provides resistance to segregation and/or dusting of the alloying powder but also provides lubricity during compaction, increasing the powder compressibility without increasing die ejection forces.
The use of powder metallurgical techniques in the production of metal parts is well established. In such manufacturing, iron or steel powders are often mixed with at least one other alloying element, also in particulate form, followed by compaction and sintering. The presence of the alloying element permits the attainment of strength and other mechanical properties in the sintered part at levels which could not be reached with unalloyed iron or steel powders alone.
The alloying ingredients that are normally used in iron-based powder mixtures, however, typically differ from the base iron or steel powders in particle size, shape, and density. For example, the average particle size of the iron-based powders normally used in the manufacture of sintered metal parts is typically about 70-100 microns. In contrast, the average particle size of most alloying ingredients used in conjunction with the iron-based powders is less than about 20 microns, most often less than 15 microns, and in some cases under 5 microns. Alloying powders are purposely used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solid-state diffusion during the sintering operation. This extremely fine size, together with the overall differences between the iron-based and alloying powders in particle size, shape, and density, make these powder mixtures susceptible to the undesirable separatory phenomena of segregation and dusting.
In general, powder compositions are prepared by dry-blending the iron-based powder and the alloying powder. Initially, a reasonably uniform blend is attained, but upon subsequent handling of the mixture, the difference in morphology between the two powder components immediately causes the two different powders to begin to separate. The dynamics of handling the powder mixture during storage and transfer cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix. The normal forces of gravity, particularly where the alloying powder is denser than the iron powder, cause the alloying powder to migrate downwardly toward the bottom of the mixture's container, resulting in a loss of homogeneity of the mixture (segregation). On the other hand, air currents which can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can, in the phenomenon known as dusting, escape the mixture entirely, resulting in a decrease in the concentration of the alloy element.
Various organic binders have been used to bind or "glue" the finer alloying powder to the coarser iron-based particles to prevent segregation and dusting. For example, U.S. Pat. No. 4,483,905 to Engstrum teaches the use of a binding agent that is broadly described as being of "a sticky or fat character" in an amount up to about 1% by weight of the powder composition. U.S. Pat. No. 4,676,831 to Engstrum discloses the use of certain tall oils as binding agents. Also, U.S. Pat. No. 4,834,800 to Semel discloses the use of certain film-forming polymeric resins that are insoluble or substantially insoluble in water as binding agents. These binders are effective in preventing segregation and dusting, but like any of the other organic binders used by the prior art, they can adversely affect the compressibility of the powder even when present in only small amounts.
The "compressibility" of a powder blend is a measure of its performance under various conditions of compaction. In the art of powder metallurgy, a powder composition is generally compacted under great pressure in a die, and the compacted "green" part is then removed from the die and sintered. It is recognized in this art that the density (and usually the strength) of this green part vary directly with the compaction pressure. In terms of "compressibility", one powder composition is said to be more compressible than another if, at a given compaction pressure, it can be pressed to a higher green density, or alternatively, if it requires less compaction pressure to attain a specified green density.
It has been found that, although the green density generally increases with the compaction pressure, the relationship is not linear; the rate of density increase levels off significantly above compaction pressures of about 30-40 tsi as the attainable density thereafter begins to approach its theoretical maximum asymptotically. Moreover, the precise degree of change in the density-pressure curve varies with the powder composition. This "leveling-off" phenomenon is more pronounced in binder-containing powder compositions of the prior art, for example, than in their unbonded counterpart compositions. Therefore, although the bonded compositions are generally more compressible than their unbonded counterparts at compaction pressures below about 30 tsi, they are less compressible at higher compaction pressures, above about 40 tsi. Depending on the particular composition, the "cross-over" point at which the bonded and unbonded compositions exhibit equivalent compressibility occurs at a compaction pressure in the range of about 30-40 tsi. Because retaining high green density is important in most powder metallurgical applications, such a decrease in compressibility at the higher compaction pressures, which usually provide the best density characteristics, can be a significant disadvantage.
Metal powder compositions are also generally provided with a lubricant, such as a metal stearate or synthetic wax, in order to facilitate ejection of the compacted component from the die. The friction forces that must be overcome in order to remove a compacted part from the die, which generally increase with the pressure used to compact the part, are measured as the "stripping" and "sliding" pressures. The lubricants reduce these pressures, but the presence of the lubricants also adversely affects compressibility. Although the compressibility of bonded powder compositions can be increased by reducing the amount of lubricant used, the resulting decrease in lubricity can cause unacceptably large increases in the ejection forces, which can result in scoring of the die, loss of die life, and imperfections in the surface of the compacted part.
Accordingly, there remains a need for a binder that permits the bonded powder composition to achieve compressibility equivalent to that of unbonded compositions, that preferably permits the reduction in the amount of lubricant content by the amount of the binder incorporated into the composition, and that at the same time maintains resistance to dusting and segregation.